1. Field of Invention
The invention relates to a multibeam scanner that scans a photosensitive medium with a plurality of laser beams, thereby exposing an image on the photosensitive medium.
2. Description of Related Art
A laser beam scanner (a single beam scanner) deflects a laser beam by a deflector, such as a polygon mirror, to a photosensitive medium and forms an image with scanning lines on the photosensitive medium. (Herein, the operation of forming one scanning line is referred to as xe2x80x9cscanning operationxe2x80x9d.)
Conventionally, the laser beam scanner comprises a beam detector that detects the laser beam at a beam detection position, before exposing the image based on image data, and outputs a detection signal (called a BD signal). Specifically, the beam detector receives the laser beam from a photodiode, converts the received laser beam to an output voltage, and amplifies the voltage by an amplifier. The beam detector detects the laser beams and outputs the BD signal, only if the amplified voltage is larger than a reference level. After a predetermined time period has elapsed since the beam detector outputs the BD signal, the laser beam scanner determines that a scan start time sets in, and starts to modulate the laser beam based on the image data. As a result, the laser beam scanner can always start the scanning operation from a predetermined scan start position.
One particular multibeam scanner comprises a plurality of laser diodes for emitting a plurality of laser beams and forms an image by using the plurality of laser beams. This multibeam scanner also comprises a beam detector for detecting each of the laser beams and outputting a BD signal for each laser beam, and therefore sets a scan start time for each laser beam.
FIG. 9 shows how the multibeam scanner, having two laser diodes LD1, LD2, forms beam spots BS1, BS2 on a photosensitive drum 77 with laser beams LB1, LB2, respectively. Under optic constraints, the laser diodes LD1, LD2 are arranged with a certain space therebetween. As shown in FIG. 9, this space is distance L1 along a main-scanning direction. Thus, there is a time lag between the laser beams LB1 and LB2 until reaching a certain position along the main-scanning direction. The distance L1 corresponds to the time lag between the laser beams LB1 and LB2 until reaching the certain position along the main-scanning direction. In other words, there is the time lag between the scan start times of the laser beams LB1 and LB2. A distance L2 along a sub-scanning direction corresponds to a distance in resolution. The photodiode, used in the beam detector, requires some operating time before outputting the BD signal. It is thus difficult to successively output the BD signals against both of the laser beams LB1, LB2 that scans with a very short time lag therebetween.
In order to overcome such a drawback, the conventional multibeam scanner stores, in a memory, the time lag corresponding to the distance L1. This time lag has previously been determined and input in the memory during manufacturing. The multibeam scanner detects only the laser beam LB1, so that the beam detector outputs the BD signal for the laser beam LB1. Then, the multibeam scanner determines the scan start time of the laser beam LB2 after the time lag from the scan start time of the laser beam LB1.
However, there are always some errors in the dimensions of the polygon mirror, and the placement of laser diodes in every multibeam scanner that uses a polygon mirror and laser diode. Accordingly, the time lag between scan start times of the laser beams varies with multibeam scanners. The time lag thus has to be determined and stored in the memory on each multibeam scanner during manufacturing. Also, it is time and labor consuming to adjust the time lag for every multibeam scanner.
In addition, the multibeam scanner is not always placed at a constant ambient temperature. The operating temperature (especially, the inner temperature) of the multibeam scanner rises under service conditions, for example, due to heat generated by heat-producing components, such as driving motors and a heater, and heat from laser beam emission. The change in such environmental temperatures may alter the properties of the polygon mirror surfaces (such as a shape and a reflectivity), and thermal expansion or shrinkage in various lenses of the optical system. Even if the time lag is adjusted in the above-explained manner, the temperature change may cause the time lag stored in the memory to deviate from the actual time lag between the scan start times of the laser beams.
For the above reasons, it is impossible for the conventional multibeam scanner to define the scan start time for each laser beam with high precision, and therefore, impossible to provide excellent print quality.
In various exemplary embodiments, the invention provides a multibeam scanner of good print quality by correcting a time lag between scan start times of laser beams.
In various exemplary aspects of a multibeam scanner for scanning an image area, the multibeam scanner comprises a plurality of beam emitting points arranged with predetermined spaces, the plurality of beam emitting points emitting a plurality of laser beams reaching the image area with respective time lags corresponding to the predetermined spaces; a time lag determination unit that determines at least one of the time lags based on the predetermined spaces; and a scan controller that controls the plurality of beam emitting points successively to start emitting the laser beams in the image area at respective scan start times, the scan start times having the determined at least one of the time lags therebetween.
According to the invention, the multibeam scanner further comprises a photodetector that detects any one of the laser beams at a predetermined beam detection position. The plurality of beam emitting points are controlled to not emit the laser beams between the photodetector and the image area. To scan the image area, the photodetector detects a first laser beam first reaching the image area at the predetermined beam detection position, prior to the image area, and determines the scan start time of the first laser beam. Then, the scan controller starts controlling the first beam emitting point at the determined scan start time to emit the first laser beam in the image area. The scan controller further controls the nth beam emitting point to emit the nth laser beam in the image area with the calculated time lag between scan start times of the nxe2x88x921th laser beam and the nth laser beam.
Particularly, the multibeam scanner of the invention comprises a deflector that comprises a plurality of deflection surfaces for deflecting the laser beams to the photodetector and to the image area, and rotates in a predetermined direction. The time lag determination unit controls the plurality of beam emitting points to emit the laser beams successively during a predetermined determination time. The photodetector detects each of the laser beams deflected by one deflection surface after the deflector rotates a predetermined number of turns. The time lag determination unit determines the time lag based on the times of detecting the laser beams during the predetermined determination time.
According to another exemplary aspect of the invention, there is provided a method for scanning an image area, the method comprising the steps of emitting a plurality of laser beams that reach the image area with respective time lags therebetween; determining at least one of the time lags; and controlling the plurality of laser beams to start emitting in the image area with the determined at least one of the time lags therebetween.
The multibeam scanner of the invention can start scanning the image area with the plurality of laser beams from the same scan start position by correcting any variations between the scan start times of the laser beam. As a result, the multibeam scanner can provide images with no loss of image quality, as described above.